1. Field of the Invention
The present invention relates to carbon nanotubes, specifically to methods for manufacturing multi-wall carbon nanotubes having tubular structures consisting mainly of carbon located within carbon nanotubes.
2. Description of the Related Art
Single-wall and multi-wall carbon nanotubes (CNT) are known to the art, and it is expected that they will become a useful material in various fields. Further, CNTs have also been studied wherein the interior (within the tube) of the CNT has been filled with various materials. For example, Japanese Unexamined Patent Application Publications Nos. 2002-97009 and 2002-97010 disclose hybrid single-wall CNTs having a structure (also termed peapod structure) whereby C60 etc. fullerenes are linked in a chain shape within the tube of single-wall CNTs.
A technique known to the art is to maintain single-wall CNTs having a peapod structure (hereafter referred to simply as peapods) for about 24 hours at 1100° C. or above, the C60 molecules contained therein fusing, and nested tubes being formed (Brian W. Smith, David E. Luzzi, Chemical Physics Letters 321 (2000) 169-174).
However, in this type of method of treating peapods over long periods, at temperatures over 1000° C. and for 12 hours or more, low numbers of nested tubes are produced. Further, in this method, virtually the same degree of thermal energy is applied to the entirety of the peapods. As a result, processing to cause fusion of the C60 cannot be performed on only a portion of the peapods (that is, selecting the locations).
Moreover, it has been reported that irradiating the peapods with electron beams having an accelerating energy of up to 100 keV for 15 minutes causes fusion of the C60 molecules therein, forming capsules (David E. Luzzi, Brian W. Smith, Carbon 38 (2000) 1751-1756).
However, the fusion of the C60 does not progress adequately in the capsules formed by electron beam irradiation. As a result, defects (structural disturbance, etc.) are relatively common. It might be considered that extending the electron beam irradiation time could cause the fusion of the C60 to progress further. However, as the above report has noted, damage (structural damage) to the CNTs forming the exterior tube of the peapods is observed when, under conventional conditions, the irradiation with electron beams having an accelerating energy of 100 keV is continued (for 5 minutes or longer, for example). The damage to the exterior tubes tends to increase as the length of electron beam irradiation time increases. Further, manufacturing efficiency and energy efficiency fall as the irradiation time is extended, and manufacturing costs consequently increase.